are you talking about die-cut letters that come in little plastic packs and you inventory them in a special box ? .... you do the " Old School Style " application where you lay down 1 letters at a time on shirts ?
If so, you will do great with these types of letters in a retail store, customers like to watch.
Just make sure your street retail store is NOT next door to the guy that makes Pizza in his store front..... he will defenetly steel the show.
btw, Josh provided good resources.